Ion beam implanters are used to implant or "dope" silicon wafers with impurities to produce n or p type extrinsic materials. The n and p type extrinsic materials are utilized in the production of semiconductor integrated circuits. As its name implies, the ion beam implanter dopes the silicon wafers with a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers. If p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium will be implanted.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion beam implanter includes beam forming and shaping structure extending between the ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
For high current ion implanters, the wafers at the implantation station are mounted on a surface of a rotating support. As the support rotates, the wafers pass through the ion beam. Ions traveling along the beam path collide with and are implanted into the rotating wafers. A robotic arm withdraws wafers to be treated from a wafer cassette and positions the wafers on the wafer support surface. After treatment, the robotic arm removes the wafers from the wafer support surface and redeposits the treated wafers in the wafer cassette.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to interior surfaces of the ion beam neutralization apparatus and to the interior walls and wafer support of the implantation apparatus. Contaminant materials include undesirable species of ions generated in the ion source, that is, ions having the wrong atomic mass.
Another source of contaminant materials results from operating the implanter to implant different species of ions in consecutive implants. It is common practice to use the same implanter for implants utilizing different ions. For example, the implanter may be utilized to implant a quantity of wafers with boron ions having an AMU of 11 (atomic mass units). The boron implant may be followed by an implant of arsenic ions having an AMU of 75. Such consecutive implants with different ion species may lead to contamination of the second implant wafers with ions from the first implant. This is referred to as "cross specie contamination."
Another contaminant is photoresist material. Photoresist material is coated on the wafer surfaces prior to ion beam treatment of the wafer and is required to define circuitry on the completed integrated circuit. As ions strike the wafer surface, particles of the photoresist coating are dislodged from the wafer and settle on the wafer support surface or adjacent interior surfaces of the beam forming and shaping structure.
Over time, the contaminant materials build up on the interior surfaces of the ion beam implanter and on the wafer support surface and decrease the efficiency of the ion beam implanter and the quality of the treated wafers. As the contaminant materials build up on the implanter component surfaces, upper layers of contaminant materials flake off or are dislodged by ions which strike the contaminant materials, creating discharges and contaminating the implantation of the wafers. Some of the dislodged contaminant material moves along the beam path to the implantation station and is implanted in the wafers. Such contaminant material changes the electrical properties of the wafers. Even a small amount of contaminant material may render the implanted wafers unsuitable for their intended purpose in the manufacture of integrated circuits.
Additionally, buildup of contaminant materials on the interior surfaces of the ion implanter will reduce the efficiency of the ion beam neutralization apparatus. The ion beam neutralization apparatus (or "electron shower") introduces low energy electrons in the vicinity of the beam and wafer to (1) lower the beam potential, and (2) neutralize the charge deposited upon the wafer surface through arrival of positive ions and emission of secondary electrons.
These electrons may be generated by secondary electron emission (termed secondary emission shower). A buildup of contaminant on the target from which secondary electrons are emitted, or on other surfaces to which these electrons can migrate, alters the number and energy distribution of the electrons produced by the neutralization apparatus.
Neutralizing electrons can also be generated by extracting them from a plasma (termed plasma shower). Though no target is required for the plasma shower, the buildup of contaminant materials on surfaces of the neutralization apparatus will also alter the number and energy distribution of electrons, and hence degrade shower operation.
The contaminants deposited on the implanter interior surfaces must be periodically removed. Removing contaminant materials from the beam forming and shaping structure and the wafer support requires disassembly of the ion beam implanter. The contaminated components are removed from the implanter and carried to a cleaning station since certain dopant materials are toxic. Component surfaces are scrubbed with solvents and abrasives to remove the contaminant materials. The implanter is then reassembled and tested prior to resuming wafer treatment.
This cleaning procedure represents a significant economic cost in terms of implanter down time. In addition to the time required for cleaning the components, reassembly of the implanter is a slow process. Precise alignment of the implanter components must be achieved for proper operation of the implanter. Additionally, the vacuum in the interior region of the implanter must be reestablished prior to operation. Finally, it is standard operating procedure not to allow a production run on an implanter that has been disassembled until it is requalified by implanting test wafers and evaluating the wafers.